It is old to extend the ground area which is contacted by a track surrounding the drive-wheel of a tractor by using a plurality of associated non-driven wheels carried by a special sub-frame mounted on the tractor, the non-driven wheels being disposed ahead and behind the axis of the drive-wheel, with the track extending around the drive-wheel, as well as the idler wheels, to thereby increase the ground area contacted by the power-driven ground engaging member, the track.
It is also old to distribute the weight of a tractor over a larger area by spreading the weight throughout the longer distance of the ground-engaging track. All of the above modifications of tractors, however, have always involved the need for a sub-assembly mounted directly on the tractor. Such sub-assemblies are necessarily relatively costly and heavy and, therefore, substantially increase the expense and over-all weight of the tractor. Most, if not all, experience tracking problems, are supported by the tractor frame and, in general, have not proved successful. As a consequence, there is still a definite need for an inexpensive but effective conversion unit for converting a wheel-driven tractor into a track-driven vehicle. This is particularly true because it is a recognized fact that traction crawler-type tractors are more effective than rubber-tired tractors, when used on soft ground or under muddy soil conditions.